1. Field of the Invention
The present invention relates to a working apparatus and calibration method thereof. In particular, the present invention relates to calibration of coordinates a working apparatus which controls a working unit having a machine using an image captured by an imaging means.
2. Description of the Related Art
Regarding assembly work and inspection using robotic arms, there is a practice in which a fixed camera detects the position and orientation of a work subject, and a robotic arm approaches and performs predetermined work. For this, a camera parameter which is a transform function between world coordinates and image coordinates, and forward and inverse kinematics that are transform mapping between robot sensor information and world coordinates must be known in advance. Because there is image strain originating from image capture in an image, and also because there is a small deviance due to mechanical error and aging, it is desirable to correct deviances over a wide range in which the robot does work with high precision. Such correction work is called calibration.
In actual calibration work, the end tip of the robot arm is designated as the work reference point, the robot arm is moved to a plurality of points, and the position of each work reference point in world coordinates is calculated from robot sensor information. Further, the positions of the work reference points in world coordinates are determined from an image taken by a fixed camera, and correction of position deviation is executed by transforming these into world coordinates.
In the calibration work mentioned above, the work reference points may not be able to be directly confirmed in the image based on the orientation of the robot arm and position of the fixed camera. For this reason, a method of attaching a flat calibration jig plate fixed with high contrast markers to the end tip of the robot arm and executing calibration is disclosed in, for example, Japanese Patent Laid-Open S64-2889 (hereafter, cited reference 1), Japanese Patent Laid-Open 3402021 (hereafter, cited reference 2), etc. Several publicly known configurations of markers such as lattice, rectangle, polygon, or circular arrangements are used.
The flat calibration jig plate mentioned above has several problems such as those mentioned below. That is,
(1) The publicly disclosed calibration methods do not make the assumption that a large portion of the indicator surface of the calibration jig is blocked. However, in the case of a robot arm with multiple degrees of freedom, it is possible that a large part of the calibration jig plate is blocked by the robot arm, depending on its position and orientation. Calibration cannot be executed in such a case using the calibration jig plate of the above example, and in order to avoid this the angle of the calibration jig plate must continually be changed, the fixed camera must be set in a location where blocking does not occur, or the calibration jig plate must be enlarged. However, because a large calibration jig plate can easily interfere with a robot arm or the floor, limiting the freedom of movement of operation of the robot arm, it is not suitable as a calibration jig.(2) When the observational angle of the calibration jig plate becomes acute due to the position and orientation of the robot arm, the estimated precision can decrease significantly because the markers become difficult to observe.(3) Conventionally, the positions and orientations of the markers are detected, and the work reference points of the robot arm are derived as relative positions to the markers. Because there is a predetermined distance between the markers and the work reference points of the robot arm, if there is an error in the estimation of the positions and orientations of the markers, an error in the calculation of operation reference points can become amplified.(4) While the work precision can be raised by using multiple cameras, an increase in the number of cameras makes avoiding problems (1) and (2) mentioned above more difficult.
As a means of solving a part of the disadvantages of the flat plate markers, markers with a three-dimensional shape can be cited. A method of gluing concentric circular markers with height differences to a work subject and measuring them with a camera, and deriving the relative position and orientation of the work subject is disclosed in Japanese Patent Laid-Open 2616225 (hereafter, cited reference 3) and Japanese Patent Laid Open H4-313106. However, as there is a necessity for the camera to be positioned somewhat directly in front of the markers using this method, this method is not suited to a case in which a jig is simultaneously measured using a plurality of cameras from a wide range of angles.
Further, Japanese Patent Laid-Open H11-189393 (hereafter cited reference 5) and Japanese Patent Laid-Open 2003-28614 (hereafter cited reference 6) mention methods using markers in a radial pattern. These methods use the fact that the pattern in the central part of the markers in a radial pattern are unchanging with respect to expansion and reduction, detect the center of the markers in a radial pattern using a template matching method, and detect the position of the work subject. However, cited references 5 and 6 above do not assume the case in which the central part of the markers is hidden and cannot be captured, and the problem (1) mentioned above can occur.